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Our Cosmic Origins

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Our Cosmic Origins Chapter 5 Our cosmic origins “In the beginning, the Universe was created. This has made a lot of people very angry and has been widely regarded as a bad move”. Douglas Adams, in The Restaurant at the End of the Universe “Oh no: he’s falling asleep!” It’s 1997, I’m giving a talk at Tufts University, and the legendary Alan Guth has come over from MIT to listen. I’d never met him before, and having such a luminary in the audience made me feel both honored and nervous. Especially nervous. Especially when his head started slumping toward his chest, and his gaze began going blank. In an act of des- peration, I tried speaking more enthusiastically and shifting my tone of voice. He jolted back up a few times, but soon my fiasco was complete: he was o↵in dreamland, and didn’t return until my talk was over. I felt deflated. Only much later, when we became MIT colleagues, did I realize that Alan falls asleep during all talks (except his own). In fact, my grad student Adrian Liu pointed out that I’ve started doing the same myself. And that I’ve never noticed that he does too because we always go in the same order. If Alan, I and Adrian sit next to each other in that order, we’ll infallibly replicate a somnolent version of “the wave” that’s so popular with soccer spectators. I’ve come to really like Alan, who’s as warm as he’s smart. Tidiness isn’t his forte, however: the first time I visited his office, I found most of the floor covered with a thick layer of unopened mail. I pulled up a random envelope as an archaeological sample, and found that it was postmarked over a decade earlier. In 2005, he cemented his legacy by winning the prestigious prize for the messiest office in Boston. 93 94 CHAPTER 5. OUR COSMIC ORIGINS Figure 5.1: Andrei Linde (left) and Alan Guth (right) at a Swedish crayfish party, blissfully unaware that I’m photographing them and that they’ll need to dress di↵erently to collect the prestigious Gruber and Milner prizes, which recognize them as the two main architects of inflation. 5.1 What’s wrong with our Big Bang? But this isn’t Alan’s only achievement. Back around 1980, he learned from the physicist Bob Dicke that there are serious problems with the earliest stages of Alexander Friedmann’s version of the Big Bang model, and proposed a radical solution that he called “inflation”1. As we’ve seen in the last two chapters, extrapolating Friedmann’s expanding-universe equations backward in time was extremely successful, accurately explaining why distant galaxies are flying away from us, why the cosmic microwave background radiation exists, how our lightest atoms originated, and many other observed phenomena. Let’s go back in time to near the frontier of our knowledge, to an instant when our Universe was expanding so fast that it would double its size during the next second. Friedmann’s equations tell us that before this, our Universe was even denser and hotter, without limit. That, in particular, there was a beginning of sorts one third of a second earlier, when the density of our Universe was infinite, and everything was flying away from everything else with infinite speed. 1Few important scientific discoveries are made by one person alone, and the discovery and development of inflation is no exception, with important contributions by Alan Guth, Andrei Linde, Alexei Starobinski, Katsuhiko Sato, Paul Steinhardt, Andy Albrecht, Viatch- eslav Mukhanov, Gennady Chibisov, Stephen Hawking, So-Young Pi, James Bardeen, Michael Turner, Alex Vilenkin and others. You’ll find interesting historical chronicles of this in many of the inflation books in the “Further Reading” section at the end of this book. 5.1. WHAT’S WRONG WITH OUR BIG BANG? 95 Following in Dicke’s footsteps, Alan Guth carefully analyzed this story of our ultimate origins, and realized that it seemed awfully contrived. For example, it gives the following answers to four of our cosmic questions from the beginning of Chapter 2: Q: What caused our Big Bang? • A: There’s no explanation — the equations simply assume it happened. Q: Did our Big Bang happen at a single point? • A: No. Q: Where in space did our Big Bang explosion happen? • A: It happened everywhere, at an infinite number of points, all at once. Q: How could an infinite space get created in a finite time? • A: There’s no explanation — the equations simply assume that as soon as there was any space at all, it was infinite in size. Do you feel that these answers settle the matter, elegantly laying all your Big Bang questions to rest? If not, then you’re in good company! In fact, as we’ll see, there’s even more that Friedmann’s Big Bang model fails to explain. 5.1.1 The Horizon Problem Let’s analyze more carefully the third question from our list above. Figure 5.2 illustrates the fact that the temperature of the cosmic microwave background radiation is almost identical (agreeing to about five decimal places) in di↵erent directions in the sky. If our Big Bang explosion had happened significantly ear- lier in some regions than others, then di↵erent regions would have had di↵erent amounts of time to expand and cool, and the temperature in our observed cos- mic microwave background maps would vary from place to place not by 0.002% but by closer to 100%. But couldn’t some physical process have made the temperatures equal long after the Big Bang? After all, if you pour cold milk into hot co↵ee as in Fig- ure 5.2, you won’t be surprised if everything mixes to a uniform lukewarm tem- perature before you drink it. The catch is that this mixing process takes time: you need to wait long enough for milk and co↵ee molecules to move through the liquid and mix. In contrast, the distant parts of our Universe that we can see haven’t had time for such mixing (Charles Misner and others first pointed this out back in the sixties). As illustrated in Figure 5.2, the regions A and B that we see in opposite directions of the sky haven’t had time to interact at all: even information traveling at the speed of light couldn’t have made it from A to B yet, since light from A is only now reaching the half-way point (where we’re located). This means that Friedmann’s Big Bang model o↵ers no explanation whatsoever for why A and B have the same temperature. So regions A and B seem to have had the same amount of time to cool since our Big Bang, which must mean that they independently underwent a Big Bang explosion at almost exactly the same time, without any common cause. 96 CHAPTER 5. OUR COSMIC ORIGINS A B Figure 5.2: Whereas the molecules of hot co↵ee and cold milk have ample time to interact with each other and reach the same temperature, the plasma in regions A and B have never had time to interact at all: even information traveling at the speed of light couldn’t have made it from A to B yet, since light from A is only reaching us co↵ee drinkers at the half-way point today. The fact that the plasma at A and B nonetheless have the same temperature is therefore an unexplained mystery in Friedmann’s Big Bang model. To better understand Alan Guth’s puzzlement over this, imagine how you’d feel if you checked your email and found a lunch invitation from a friend. And then realized that every other friend of yours has also sent you a separate email inviting you for lunch. And that every single one of these emails were sent to you at the exact same time. You’d probably conclude that this was some sort of conspiracy, and that all the emails had a common cause. Perhaps your friends had communicated amongst themselves and decided to throw you a surprise party, say. But to complete the analogy with Alan’s Big Bang puzzle, where the regions A, B etc. correspond to your friends, imagine that you know for a fact that your friends have never met, have never communicated with each other, and have never had access to any common information before they sent you their emails. Then your only explanation would be that it was all a crazy fluke coinci- dence. Too crazy to be plausible, in fact, so you’d probably conclude that you’d made an incorrect assumption somewhere, and that your friends had somehow managed to communicate after all. This is exactly what Alan concluded: it couldn’t just have been a crazy fluke coincidence that infinitely many separate regions of space underwent Big Bang explosions all at once — some physical mechanism must have caused both the exploding and the synchronizing. One unexplained Big Bang is bad enough; an infinite number of unexplained Big Bangs in perfect synchronization strains credulity. This is known as the “Horizon Problem”, because it involves what we see on our cosmic horizon, in the most distant regions we can observe. As if this weren’t bad enough, Bob Dicke had told Alan of a second problem for Friedmann’s Big Bang that he called the “Flatness Problem”. 5.1.2 The Flatness Problem As we saw in the last chapter, we’ve measured our space to be flat to high accuracy.
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